Incremental position encoders utilize a scale structure that allows the displacement of a readhead relative to a scale to be determined by accumulating incremental units of displacement, starting from an initial point along the scale. Such encoders are suitable for certain applications, particularly those where line power is available. However, in certain applications, such as those where encoders are used in low power consumption devices, it is more desirable to use absolute position encoders. Absolute position encoders provide a unique output signal, or combination of signals, at each position along a scale. They do not require continuous accumulation of incremental displacements in order to identify a position. Thus, absolute position encoders allow various power conservation schemes. A variety of absolute position encoders are known, using various capacitive, inductive or optical sensing technologies.
One of the most important figures of merit for an absolute encoder is (range/resolution), that is, the maximum allowed absolute measuring range for a device in comparison to its meaningful measurement resolution and/or accuracy. This may be referred to as its “range-to-resolution ratio.”
Some encoders achieve a high range-to-resolution ratio by using a number of binary code tracks that run in parallel along a scale. The range of this technique is generally limited by the width of the scale, which determines the number of binary tracks that may be fabricated. Furthermore, crude binary sensing generally limits the resolution. This technique is generally not optimum for narrow scales, which are desirable in a compact encoder. It will be appreciated that the least significant bit (LSB) binary code track may be thought of as a “fine wavelength” incremental track, since it repeats at the “fine” spatial resolution of the LSB and provides only incremental displacement information (that is, it provides only periodic non-absolute signal), unless it is used in combination with tracks that provide more significant code bits. This is characteristic of the fine wavelength tracks that are used in most high-resolution absolute encoders (e.g., those that provide resolution on the order of microns). Thus, the fine wavelength track (fine track) may also be referred to as an incremental track in many absolute encoders.
In comparison to an “all binary” technique, some encoders enhance the resolution of the fine track by using techniques that provide an analog signal related to that wavelength, and then measuring that analog signal to within some fraction of its range, to provide resolution that is finer than the fine wavelength, and thereby extend the range-to-resolution ratio of an absolute encoder. This is typically referred to as signal interpolation, and the ratio of the fine wavelength to the resulting measurement resolution is typically referred to as the interpolation ratio. Depending on the technology used, and the level of expenditure used to provide the precision components and assembly that govern the signal-to-noise (S/N) ratio, practical signal interpolation ratios of up to 100, 300, or even 1000 or more are possible. However, generally speaking, an interpolation ratio greater than approximately 100 may require significant additional expense for the required precision components and assembly.
Some encoders abandon binary tracks and use signal interpolation on an additional scale track that is coarser than the fine track. Such a track may be referred to as an absolute scale track (absolute track). It will be appreciated that such signal interpolation must have resolution and repeatability within plus and minus one half of the fine wavelength, in order to resolve the ambiguity of the periodic signals provided by the fine track. Some encoders use an absolute track that varies monotonically (e.g., linearly) over the entire measurement range. However, assuming a fine track wavelength on the order of 20-100 microns, and an interpolation ratio on the order of 100, such an absolute track alone would bring the associated absolute measuring range up to only 2-10 millimeters, which is of limited utility.
To overcome this limitation, some encoders use at least two additional absolute tracks that have significantly longer spatial wavelengths than the fine track. Their wavelengths may be conveniently referred to as absolute wavelengths and/or medium wavelengths and/or coarse wavelengths, in order to distinguish them from the fine wavelength and/or emphasize their function and their relative spatial wavelength relationships. As one example, using known sensing techniques (e.g., optical sensing techniques), periodic analog signals (e.g., sinusoidal signals or similar processed outputs, or the like) are derived from two medium-wavelength absolute tracks (also referred to as medium tracks) that have slightly different medium wavelengths. According to known relationships, the spatial phase difference between the two analog signals changes by 360 degrees over a distance that is proportional to the product of the medium wavelengths and inversely proportional to the absolute value of their difference. This distance may be referred to as a synthetic wavelength, which is approximately the absolute measurement range of the device if there are no coarser wavelength tracks. The phase difference between the signals from medium tracks can be used in conjunction with the known synthetic wavelengths to provide the absolute position to a coarse resolution. This may be referred to as the coarse position. The coarse position resolution and/or accuracy must be within approximately plus and minus one half of one of the medium wavelengths, in order to resolve the ambiguity of the periodic signal(s) provided by the medium track, in order to reliably identify a particular period of the medium wavelength corresponding to the coarse position. The periodic signal(s) from that medium track may then be interpolated within that particular period of the medium wavelength to provide the absolute position to a medium resolution that is better than the coarse resolution. This may be referred to as the medium position. The medium position resolution and/or accuracy must be within approximately plus and minus one half of one fine wavelength, in order to resolve the ambiguity of the periodic signal(s) provided by the fine track, in order to reliably identify a particular period of the fine wavelength corresponding to the medium position. The periodic signal from the fine track may then be interpolated within that particular period of the fine wavelength to provide the absolute position of a device with the ultimate fine resolution and/or accuracy. The foregoing technique is generally known and additional detail regarding various related encoder configurations and/or signal processing is readily available in various absolute encoder and absolute interferometer patents. The foregoing technique may be referred to as a Synthetic Coarse Wavelength Absolute Measurement Technique (SCWAM technique).
U.S. Pat. Nos. 3,882,482, 5,965,879, 5,279,044, 5,886,519, 5,237,391, 5,442,166, 4,964,727, 4,414,754; 4,109,389; 5,773,820; and 5,010,655, disclose various encoder configurations and/or signal processing techniques relevant to absolute encoders, including those outlined above, and are hereby incorporated herein by reference in their entirety. However, the prior art fails to teach configurations which provide certain combinations of range-to-resolution ratio, high resolution, compact size, robustness, and cost desired by users of absolute encoders. It will be appreciated that extending the range-to-resolution ratio without increasing the width of the scale, detector, and/or other encoder components is particularly difficult. Improved configurations of absolute encoders that provide such combinations would be desirable.